PROJECTSUMMARY Vitamin D repletion is linked to improved muscle mitochondrial function, lipid deposition and preservation;? however, while vitamin D insufficiency is common in cancer, the mechanistic effects of vitamin D on muscle metabolic health in cancer patients have not been studied. This is important to address because cancer cachexiaischaracterizedbymarkedmusclewasting,anabolicresistance,ectopicfatinfiltration,mitochondrial dysfunctionandcontributestodecreasedsurvival.Withnovelstrategiestoaddressthisknowledgegap,wewill use a combination of advanced metabolic analytical approaches with complementary model systems in cell culture and human subjects to understand the biochemical and physiological mechanisms underlying cancer cachexia in relation to the role of Vitamin D in conjunction with resistance exercise (RE). By combining analysesofmusclesizeandlocaltissuehemodynamicsinvivo,metabolomicsanalysesofmuscletissueand isolated mitochondria, and changes in anabolic cell signaling, lipid metabolism and oxidative capacity of primary muscle cells in vitro, we will identify mechanisms underlying muscle response to vitamin D repletion. Our previous findings, together with data that exercise improves muscle vitamin D storage and retrieval, suggest that vitamin D repletion synergizes with RE to improve muscle metabolic function and protein synthesis. Our overall objective is to examine mitochondrial function and anabolic resistance as potential targets of action of vitamin D on muscle metabolism, size and strength in preventing the progression of cachexia. The aims of this study are to: 1) non-invasively quantify lipid redistribution, local muscle tissue metabolismandmusclemassandstrengthofcancerpatientsbeforeandafter12weeksofvitaminDrepletion withexerciseandproteinsupplementation(VitD)comparedtoexerciseandproteinsupplementationonly(Ctl);? 2) determine differences in muscle mitochondrial function in live tissue biopsied from human gastrocnemius from VitD compared to Ctl;? and 3) identify mechanisms whereby vitamin D and exercise regulate muscle anabolicsignalingandmitochondrialactivityinprimaryhumanmyotubecultures.Ourcentralhypothesisisthat vitamin D promotes muscle lipid availability for ?-oxidation in response to exercise, thereby preventing lipotoxicityinthemuscleandpotentiallyimprovinganabolicsensitivityinmuscleduringcancercachexia.The impact of this project, the first nutrition and exercise study designed as an inexpensive intervention, is to understand the effect of vitamin D on the metabolic and anabolic dynamics which underpin dysfunction in cachecticmuscle.IfvitaminDpromoteslipidpartitioning,musclemetabolicfunctionand/oranabolicsensitivity, theseadaptationswillultimatelyimprovecancertherapybycombatingcancercachexia.Further,diffuseoptical spectroscopy techniques have the potential to identify the minimum effective intervention dose for optimizing metabolichealthleadingtomorepracticalandindividualizedlifestyleprescriptionstoreducehealthcarecosts.